(1) Field of the Invention
This invention relates to a release coating composition used for a pressure sensitive adhesive tape or contact sheet, and to a pressure sensitive adhesive tape or contact sheet. More particularly, it relates to an aqueous emulsion type release coating composition used for a pressure sensitive adhesive tape or contact sheet, which has a base such as a thermoplastic plastic film. The invention further relates to a pressure sensitive adhesive tape or contact sheet.
(2) Description of the Related Art
A pressure sensitive adhesive tape or contact sheet has an adhesive layer formed on a major surface thereof and is protected by a cover sheet. When the pressure sensitive adhesive tape or contact sheet is used, the cover sheet is peeled therefrom. A surface of the cover sheet, which is placed in contact with the adhesive layer, is coated with a release coating composition for easy peeling of the cover sheet from the adhesive layer.
As examples of the release coating composition, there can be mentioned a silicone release coating composition and a non-silicone release coating composition. The silicone release coating composition includes an addition type silicone or a condensation type silicone. The silicone release coating composition exhibits excellent releasability, residual adhesion and heat resistance, but is expensive, and must be heat-cured at a high temperature and thus is difficult or even impossible to apply to a plastic film, especially a thermoplastic film.
The non-silicone release coating composition comprises, for example, a long chain alkyl pendant type polymer. The non-silicone release coating composition generally has releasability and other characteristics, which are inferior to those of the silicone release coating composition. But, a non-silicone release coating composition comprising a long chain alkyl pendant type polymer, as described in, for example, Japanese Examined Patent Publication No. 60-30355, exhibits releasability which is acceptable to a satisfying extent, and thus is widely used. Further, this non-silicone release coating composition is advantageous in that there is no need of using a special catalyst nor curing at a high temperature. Releasability can be obtained merely by coating a base film with a solution of the release coating composition in a solvent, followed by drying, and therefore, the release coating composition can advantageously be applied to a base film having a poor heat resistance such as a thermoplastic film. However, the long chain pendant type polymer is insoluble in water and must be used as a solution in an organic solvent.
However, in recent years, a deep interest is felt on problem of safety and environment, and thus, an aqueous release coating composition is eagerly desired. Proposals of an aqueous emulsion type release coating composition as an aqueous release coating composition have been made. For example, in Japanese Unexamined Patent Publication (hereinafter abbreviated to xe2x80x9cJP-Axe2x80x9d) No. H3-86,778, an aqueous release coating composition comprising an aqueous emulsion of a long chain alkyl pendant type polymer, which is a product produced by reaction of polyvinyl alcohol or a modified product thereof with an alkyl isocyanate, is proposed. In JP-A H8-176,372, an aqueous release coating composition comprising an aqueous dispersion of (i) a product produced by reaction of a saponified vinyl acetate copolymer having a specific polymerization degree and saponification degree with a long chain alkyl compound having a functional group capable of reacting with hydroxyl groups of the saponified vinyl acetate copolymer, and (ii) an acid-modified olefin copolymer with a specific acid value.
The heretofore proposed aqueous emulsion type release coating compositions have poor storage stability, i.e., when the coating compositions are allowed to stand for a long period, the long chain alkyl pendant-type polymer contained as an effective ingredient is readily precipitated or coagulated. Further, the aqueous emulsion type release coating compositions have releasability and residual adhesion, which are inferior to those of non-aqueous release coating compositions containing an organic solvent.
A primary object of the present invention is to provide an emulsion-type release coating composition exhibiting high and balanced releasability and residual adhesion, and having good storage stability.
Another object of the present invention is to provide a pressure sensitive adhesive tape or contact sheet exhibiting high and balanced releasability and residual adhesion.
In accordance with the present invention, there is provided an aqueous emulsion-type release coating composition comprising (i) a product produced by reaction of an ethylene-vinyl alcohol copolymer represented by the following formula (1): 
wherein the total of n, m and mxe2x80x2 is an integer of 300 to 3,000, the ratio of n/(n+m+mxe2x80x2) is in the range of 0.1 to 0.9, and the ratio of m/(m+mxe2x80x2) is in the range of 0.9 to 1.0, with a C8-30 alkyl isocyanate (said product is hereinafter referred to as xe2x80x9cmodified Et-VAl copolymerxe2x80x9d when appropriate), and (ii) a surface active agent which is a combination of a nonionic surface active agent having a hydrophilic-lyophilic balance (HLB) of not larger than 12, with an ionic surface active agent.
In accordance with the present invention, there is further provided a pressure sensitive adhesive tape or contact sheet, having a laminate structure composed of superposed base films or a wound base film, each of which has a layer of a pressure sensitive adhesive formed on one surface thereof, and a layer of a release coating composition formed on the other surface thereof so that the release coating composition layer is in direct contact with the pressure sensitive adhesive layer formed on one surface of the adjacent base film; said release coating composition being the above-mentioned aqueous emulsion type release coating composition.
The aqueous emulsion-type release coating composition of the present invention is characterized as containing a combination of a nonionic surface active agent having a low HLB, i.e., a HLB of not larger than 12, (hereinafter abbreviated to xe2x80x9clow-HLB surfactantxe2x80x9d when appropriate) and an ionic surface active agent (hereinafter abbreviated to xe2x80x9chigh-HLB surfactantxe2x80x9d when appropriate).
In general, in order to prepare an aqueous polymer emulsion by emulsifying the polymer with a surface active agent, a surface active agent having a HLB falling between that of water and that of the polymer is used as an emulsifier. However, the modified Et-VAl copolymer used in the present invention has an extremely hydrophobic polymer, and therefore, a surface active agent used for the preparation of an aqueous emulsion of the modified Et-VAl copolymer must be appropriately chosen. If a surface active agent having a high HLB is used, it has poor affinity for and compatibility with the modified Et-VAl copolymer, and thus, the modified Et-VAl copolymer is difficult to emulsify. In contrast, a surface active agent having a low HLB is used, it has good affinity for and compatibility with the modified Et-VAl copolymer, but the surface active agent is too hydrophilic to emulsify the modified Et-VAl copolymer.
A nonionic surface active agent having a HLB not larger than 12, i.e., a low-HLB surfactant, used in the present invention exhibits a high affinity for the modified Et-VAl copolymer. An ionic surface active agent, i.e., a high-HLB surfactant, used in the present invention exhibits a high affinity for water as ionic surface active agents generally do. The high-HLB surfactant and the low-HLB surfactant are compatible with each other. Therefore, the modified Et-VAl copolymer can be easily emulsified with a combination of the two surfactants. The thus-prepared aqueous emulsion of the modified Et-VAl copolymer is of an oil-in-water (O/W) type.
The nonionic surface active agent having a HLB of not larger than 12 preferably includes polyoxyethylene alkyl-ether type surface active agents having a HLB of not larger than 12. The polyoxyethylene alkyl-ether type surface active agents having a HLB of not larger than 12 have a high affinity for the modified Et-VAl copolymer and, when a release coating composition made therewith is coated on a film and heat-treated to be dried, undesirable bleeding of the nonionic surface active agent can be prevented. Therefore, although a surface active agent is used, good releasability achieved with the modified Et-VAl copolymer can be retained, and reduction of residual adhesion caused by the ingredients in an aqueous emulsion type release coating composition can be prevented or minimized.
As preferable examples of the polyoxyethylene alkyl-ether type surface active agent, there can be mentioned polyoxyethylene C8-20 alkyl-ethers.
The ionic surface active agents used in the present invention preferably include an anionic surface active agent and a cationic surface active agent. When any of anionic surface active agents and cationic surface active agents are used in combination with a nonionic surface active agent having a HLB of not larger than 12, the ethylene-vinyl alcohol copolymer can easily be emulsified.
The amount of the nonionic surface active agent used is preferably in the range of 10 to 60% by weight, based on the weight of the entire reaction product of the modified Et-VAl copolymer with C8-30 alkyl isocyanate. The ratio of the nonionic surface active agent/the ionic surface active agent is preferably in the range of 0.3 to 30 by weight.
Modified Et-VAl Copolymer
In the ethylene-vinyl alcohol copolymer of formula (1) used for the preparation of the modified Et-VAI copolymer, the sum of (n+m+mxe2x80x2) is an integer of 300 to 3,000, preferably 500 to 2,500. If the polymerization degree of the ethylene-vinyl alcohol copolymer is too small, releasability of the release coating composition is poor. In contrast, the polymerization degree of the copolymer is too large, dispersibility of the modified Et-VAl copolymer in water is reduced.
The ratio of n/(n+m+mxe2x80x2), namely, the ratio by mole of ethylene units in the ethylene-vinyl alcohol copolymer, is in the range of 0.1 to 0.9, preferably 0.2 to 0.6.
The ratio of m/(m+mxe2x80x2) is in the range of 0.9 to 1.0, namely, the saponification degree of the ethylene-vinyl alcohol copolymer is in the range of 90 to 100% by mole. If the saponification degree is smaller than 90% by mole, the modified Et-VAl copolymer has a low melting point and an increased affinity for an adhesive composition, and thus, the release coating composition is liable to migrate into the adhesive layer with the result of reduction of releasability.
In the C8-30 alkyl isocyanate used for the modified Et-VAl copolymer, the alkyl group has 8 to 30 carbon atoms, preferably 8 to 20 carbon atoms. A straight chain alkyl group is preferable. As specific examples of the C8-30 alkyl isocyanate, there can be mentioned octyl isocyanate, lauryl isocyanate (i.e., dodecyl isocyanate) and stearyl isocyanate (i.e., octadecyl isocyanate).
The modified Et-VAl copolymer is prepared by reacting the ethylene-vinyl alcohol copolymer of formula (1) with the C8-30 alkyl isocyanate. More specifically, the ethylene-vinyl alcohol copolymer of formula (1) is dissolved in an organic solvent such as dimethyl sulfoxide, and the alkyl isocyanate is added to the copolymer solution at an elevated temperature, for example, about 80xc2x0 C., with stirring to dissolve the alkyl isocyanate. If desired, a catalyst such as dibutyltin dilaurate is added, and the temperature of the mixture is further elevated to, for example, about 140xc2x0 C. whereby the ethylene-vinyl alcohol copolymer is reacted with the alkyl isocyanate to give the modified Et-VAl copolymer.
The amount of the alkyl isocyanate used is not particularly limited, but is usually at least 0.5 mole, preferably 0.5 to 1.5 moles and more preferably 0.6 to 1.1 moles.
The termination of the reaction can be confirmed, for example, by measuring the amount of unreacted alkyl isocyanate remaining in the reaction mixture by the infrared spectroscopic analysis.
The modified Et-VAl copolymer produced can be separated from the reaction mixture by a conventional procedure, for example, a precipitation procedure. For example, the as-obtained reaction mixture is cooled to 80xc2x0 C., and then, incorporated in a bath of an alcohol such as methanol or isopropyl alcohol in an amount of 3 to 6 times of the amount of the reaction mixture whereby the modified Et-VAl copolymer is precipitated or coagulated. The precipitated or coagulated polymer is washed with an alcohol such as methanol or isopropyl alcohol, thus being separated.
The modified Et-VAl copolymer may be used either alone or in combination. The amount of the modified Et-VAl copolymer incorporated in the release coating composition of the present invention is not particularly limited and can be varied depending upon the kind of pressure-sensitive adhesive. Usually the amount of the modified Et-VAl copolymer is in the range of 1 to 20% by weight, preferably 5 to 15% by weight, based on the weight of the aqueous emulsion-type release coating composition. When the amount of the modified Et-VAl copolymer is too small, the adhesion between the pressure sensitive adhesive tape or adhesive sheet and the protective cover sheet is reduced. In contrast, when the amount of the modified Et-VAl copolymer is too large, a precipitate of unemulsified copolymer tends to be produced in the release coating compsosition.
Nonionic Surface Active Agent
The nonionic surface active agent used in combination with an ionic surface active agent in the present invention has a hydrophilic-lyophilic balance (HLB) of not larger than 12, usually in the range of 3 to 12, preferably 5 to 10 and more preferably 6 to 10. If the HLB of the nononic surface active agent is larger than 12, the releasability and residual adhesion are reduced.
The nonionic surface active agent used includes, for example, polyoxyethylene alkyl-ether type surface active agents (including a straight chain polymer and a branched chain polymer), polyoxyethylene alkyl-phenyl-ether type surface active agents, polyoxyethylene fatty acid ester type surface active agents, polyoxyethylene polyhydric alcohol fatty acid ester type surface active agents, polyoxyethylene higher-alkylamine fatty acid ester type surface active agents, polyoxyethylene fatty acid-amide type surface active agents, fatty acid esters of polyhydric alcohols (for example, pentaerythritol, sorbitol and sucrose), and alkanolamides. The nonionic surface active agents may be used either alone or in combination.
As specific examples of the polyoxyethylene alkyl-ether type surface active agent, there can be mentioned polyoxyethylene C8-20 (preferably C8-18) alkyl ethers such as polyoxyethylene decyl ether, polyoxyethylene lauryl ether (i.e., polyoxyethylene dodecylether), polyoxyethylene tetradecyl ether, polyoxyethylene pentadecyl ether, polyoxyethylene hexadecyl ether and polyoxyethylene stearyl ether (i.e., polyoxyethylene hexadecyl ether). As specific examples of the polyoxyethylene alkyl-phenyl-ether type surface active agent, there can be mentioned polyoxyethylene C8-20 (preferably C8-18) alkyl-phenyl-ethers such as polyoxyethylene nonyl-phenyl-ether.
In the nonionic surface active agents, especially polyoxyethylene alkyl ether type surface active agents, the hydrophobic hydrocarbon group, i.e., alkyl group, has at least 8, usually 8 to 20, preferably 8 to 18 carbon atoms. The alkyl group may be either straight chain or branched, but is preferably straight chain. This is because, when a release coating composition containing a nonionic surface active agent having a straight chain alkyl group is coated on a film and is then heat-dried, bleeding of the nonionic surface active agent from the coating can be prevented or minimized, with the results that good releasability achieved with the modified Et-VAl copolymer can be retained, and reduction of residual adhesion caused by the ingredients in an emulsion type release coating composition can be prevented or minimized. Thus, polyoxyethylene alkyl ether type surface active agents, especially those having a straight chain alkyl group, are preferable as the nonionic surface active agent.
Ionic Surface Active Agent
The ionic surface active agent used in combination with the nonionic surface active agent includes an anionic surface active agent, a cationic surface active agent and an ampholytic surface active agent. The ionic surface active agent may be used either alone or in combination.
As specific examples of the ionic surface active agent, there can be mentioned alkylsulfuric acid ester salts such as sodium C10-18 alkylsulfate esters, for example, sodium laurylsulfate ester; alkyl-ether-sulfuric acid ester salts such as sodium C10-18 alkyl-ether-sulfate esters, for example, sodium lauryl-ether-sulfate ester; alkyl-diphenyl-ether-disulfuric acid salts such as sodium C10-18 alkyl-diphenyl-ether-disulfate, for example, sodium lauryl-diphenyl-ether-disulfate; alkylbenzenesulfonic acid salts such as sodium C10-18 alkylbenzenesulfonate; sulfosuccinic acid salts such as sodium sulfosuccinate; dialkylsulfosuccinic acid salts such as sodium di-C10-18 alkylsulfosuccinates; aliphatic monocarboxylic acid salts; N-acyloylglutamic acid salts; alkylnaphthalenesulfonic acid salts such as sodium alkylnaphthalenesulfonate; olefin-sulfonic acid salts such as sodium xcex1-olefin-sulfonate; sulfurized fatty acid esters; sulfurized olefins; and phosphoric acid ester salts. Of these, alkylsulfuric acid ester salts, alkyl-ether-sulfuric acid ester salts, alkyl-diphenyl-ether-disulfonic acid ester salts are preferable.
As specific examples of the cationic surface active agent, there can be mentioned alkylammonium salts such as alkylammonium halides, for example, laurylammonium chloride; alkyltrimethyl-ammonium halides; trimethylaminoethylakylamide halides; alkylpyridinium sulfuric acid salts; and alkyldimethylbenzyl-ammonium salts. Of these, alkylammonium salts are preferable.
As specific examples of ampholytic surface active agents, alkyltrimethylaminoacetic acids, alkyldiethylenetriaminoacetic acids, alkylaminopropionic acids and alkyldimethylbetaines.
Among the ionic surface active agents, anionic surface active agents and cationic surface active agents are preferable. Especially anionic or cationic surface active agents which have a hydrophobic group, such as an alkyl group, having a large number of carbon atoms, for example, about 10 to 20 carbon atoms. In the anionic surface active agents and cationic surface active agents, the structure of the hydrophobic group may have either a straight chain structure or a branched chain structure, but a straight chain structure is preferable because a hydrophobic group with a straight chain structure gives only a slight influence on the releasing properties. That is, when a release coating composition containing an anionic or cationic surface active agent having a straight chain alkyl group is coated on a film and the coating is heat-treated and dried, bleeding of the anionic or cationic surface active agent can be prevented or minimized.
The amount of the nonionic surface active agent is usually in the range of 10 to 60% by weight, preferably 10 to 50% by weight, based on the total amount of the modified Et-VAl copolymer. The amount of the ionic surface active agent is usually in the range of 2 to 30% by weight, preferably 5 to 20% by weight, based on the total amount of the modified Et-VAl copolymer. The ratio of the nonionic surface active agent/the ionic surface active agent is usually in the range of 0.3 to 30 by weight, preferably 0.5 to 10 by weight.
Emulsion Type Release Coating Composition
The aqueous emulsion type release coating composition of the present invention comprises the above-mentioned modified Et-VAl copolymer, nonionic surface active agent and ionic surface active agent.
If desired, organic solvents such as higher alcohols, for example, decyl alcohol, and petroleum hydrocarbons, may be incorporated in the release coating composition. In the case where the organic solvent is used, its amount is preferably not larger than 25% by weight based on the weight of the modified Et-VAl copolymer.
The aqueous emulsion type release coating composition is prepared by emulsifying the modified Et-VAl copolymer by using a combination of the nonionic surface active and the ionic surface active agent. More specifically. the modified Et-VAl copolymer, the nonionic surface active agent, the ionic surface active agent, an optional organic solvent and a predetermined amount of water are mixed together. By using an apparatus capable of emulsifying the mixture under a high pressure (i.e., a high-pressure emulsifier), the mixture is stirred at a high temperature, for example, about 100xc2x0 C. and at a low-rate of revolution, for example, about 100 to 200 rpm, and then, the stirring is continued at a higher rate of revolution, for example, about 400 to 700 rpm and at a high temperature, for example, about 100xc2x0 C., to prepare a pre-emulsion. The remainder of water is added to the pre-emulsion with stirring and the mixture is cooled to give the aqueous emulsion type release coating composition.
Pressure Sensitive Adhesive Tape or Contact Sheet
The aqueous emulsion type release coating composition of the present invention is applicable to various pressure sensitive tapes or contact sheets, and pressure sensitive label paper or sheets. The aqueous emulsion type release coating composition is especially suitable for a pressure sensitive adhesive tape or contact sheet having a laminate structure composed of superposed base films or a wound base film, each of which has a layer of a pressure sensitive adhesive formed on one surface thereof, and a layer of a release coating composition formed on the other surface thereof so that the release coating composition layer is in direct contact with the pressure sensitive adhesive layer formed on one surface of the adjacent base film. The aqueous emulsion type release coating composition also is suitable for a pressure sensitive adhesive label paper or sheet having a laminate structure composed of a base film having a layer of a pressure sensitive adhesive formed on one surface thereof, and a protecting sheet having a layer of a release coating composition formed on one surface thereof; the base film being superposed on the protecting sheet so that the pressure sensitive adhesive layer is in direct contact with the release coating composition layer.
The aqueous emulsion type release coating composition of the present invention is coated on a back surface (i.e, a surface to be placed in contact with an adhesive) of a pressure sensitive adhesive tape or contact sheet, or on a surface of a protecting cover sheet of a pressure sensitive label paper or sheet. The coating can be conducted by using a conventional apparatus, for example, a roll coater, a gravure coater, a Mayer bar coater or a lip coater.
The base film or sheet of the pressure sensitive adhesive tape or contact sheet, and the base sheet of the protecting cover sheet are not particularly limited, and include, for example, thermoplastic films and sheets made of polyethylene, polypropylene, polyester such as polyethylene terephthalate, and cellophane, i.e., cellulose acetate; papers such as woodfree paper, craft paper, crepe paper and glassine paper; seal-coated papers such as impregnated paper and plastic-coated paper; and cloths. The coated surface of plastic film or sheet may be surface-treated by a conventional surface-treating procedure such as corona discharge treatment or plasma treatment.